It is known that electric power systems which supply highly reactive loads are characterized by poor voltage regulation, i.e., substantial change in the magnitude of load voltage as load current increases. In a typical inductive circuit voltage magnitude and power factor both decrease as load current increases. To improve system voltage regulation power transformers are commonly provided with tap changers to counteract the tendency of voltage magnitude to change with change in load current. Since most system loads are inductive it is known also to counteract the inductive current components of system load or of particular major loads by connecting compensating capacitance in series with or in shunt across power line conductors. Fixed capacitors may be used where the load is reasonably steady and predictable.
With certain variable and erratic major loads, such as electric arc furnaces, controllable shunt capacitance has been provided by connecting rotating synchronous condensors or static capacitors directly across the load terminals in parallel with the load. The amount of capacitance must be varied as load current changes, for fixed capacitance would have the effect on no load of increasing load terminal voltage above the applied system voltage.
Several arrangements have recently been proposed for varying the net reactive current effect of fixed shunt compensating capacitors by connecting compensating inductors in parallel with the capacitors and varying the amount of reactive current traversing the compensating inductors. This may be done by varying the magnitude of the shunt inductance across each line, as in U.S. Pat. No. 3,551,799--Koppleman, or by varying the magnitude of reactive current traversing a shunt inductor of fixed inductance. One such compensating circuit utilizing fixed capacitors and inductors in parallel with means for controlling the magnitude of inductor current is described in "Electric Technology -- U.S.S.R.", Vol. 1, 1969, pgs. 46-62 (Pergamon Press, October, 1969).
By whatever means such a shunt compensating impedance is varied, it has been common practice heretofore to connect the compensating impedance directly in conductive shunt circuit relation across a power line or across the line terminals of a selected major load. We have discovered however, that by coupling the compensating impedance to the load circuit through inductive or other suitable phase shifting means the compensating impedance has an enhanced effect upon the negative phase sequence reactive components of unbalanced multiphase current. In such connection a reactive compensator acts not only to improve load power factor but also to multiply its effect in eliminating current unbalance.